This invention relates in general to the synthesis of semiconductor crystals. In a more particular aspect, this invention concerns itself with a method and apparatus for synthesizing highly pure, single crystal, indium phosphide (InP)
Highly pure, single crystal InP, as well as other III-V semiconductor compounds has become an important semiconductor material for a wide variety of technical applications. InP crystals, in particular, are especially important for use as structural materials in the fabrication of substrates used for lattice-matched fiber-optic sources and detectors, high speed integrated circuits, and high frequency microwave devices. Its large band gap (1.35 eV) and high electron mobility make it especially desirable for use as a semiconductor material, especially when employed in its highly pure form. It also finds use as a crystalline substrate for device fabrication by epitaxial deposition techniques. Unfortunately, however, it is extremely difficult to grow single crystals of InP having the high degree of purity necessary for its successful utilization as a thin film device.
A number of methods have been suggested heretofore for synthesizing crystalline InP. One method for synthesizing fairly large ingots involves the direct reaction of elemental phosphorus with elemental indium. The as-supplied products have undesirable inpurities therein and must be baked before reaction. In situ-baking of these products has been accomplished in a growth apparatus where polycrystalline indium phosphide is grown. The polycrystalline product is used in the liquid encapsulated Czochralski technique to produce single crystals. During the growth of the single crystal, a layer of B.sub.2 O.sub.3 must be applied over the growth melt so that the phosphorous is prevented from escaping and thus changing the proportions in the subsequent formation as the crystal develops.
A well known method which has proven somewhat successful in growing single crystals of InP is the so-called Bridgman technique. In this technique, a single crystal ingot of InP is grown from solution by heating the polycrystalline growing materials (elemental indium phosphorus) to a temperature above their melting point, allowing the melt to cool, and, in some manner, restricting nucleation of the solid phase to a single event. In so doing, a single grain is propagated behind a liquid/solid moving interface and, when the interface has traversed the length of the melt, a single crystal remains. This is the basis of the Bridgman technique. The technique can be carried out in a horizontal arrangement or in a vertical arrangement.
In the vertical arrangement, the molten material can be made to solidify by either lowering the crucible through a heat zone, moving the heat zone, or progressively lowering the furnace temperature. In this way, the molten material is made to solidify from the lower end of the crucible. The method suffers limitations in the growth of refractory metal crystals because of the lack of suitable crucible materials. The technique does offer advantages, however, which merit consideration. Bridgman crucibles are obtainable as standard products in a variety of sizes and materials, and the Bridgman technique is capable of producing a wide size-range of crystals from thin rods to ingots several centimeters in diameter. The shape of the crucible is selectable and the standard commercial product has obviously evolved to accommodate practical scientific requirements and economical manufacturing techniques. The Bridgman crucible is usually a tube of circular cross-section with a sharp point formed at the closed end. The bore of the crucible may be parallel or slightly tapered towards the closed end. While the sharp end is not essential, it does enhance the likelihood of point nucleation. The pointed end leads through the temperature gradient defining the effective change from the liquid to solid, and it is here that the single nucleation event is required in order to grow a single grain through the rest of the material as it solidifies. A seed may also be placed in the pointed end to provide a desired orientation.